An example of such an apparatus is proposed in Japanese Patent Publication No. Hei 7-92090. In this pressurized hydraulic fluid supply apparatus, a hydraulic fluid delivered under pressure from a first hydraulic pump is supplied through a plurality of operation valves to a plurality of hydraulic actuators on one side and a hydraulic fluid delivered under pressure from a second hydraulic pump is supplied through a plurality of operation valves to a plurality of hydraulic actuators on the other side. The pressurized hydraulic fluid supply apparatus has a flow merge/separation valve to merge or separate the hydraulic fluid from the first hydraulic pump and the hydraulic fluid from the second hydraulic pump.
In the flow separation state, the pressurized hydraulic fluid supply apparatus delivers the pressurized fluid from the first hydraulic pump and the pressurized fluid from the second hydraulic pump individually to the actuators on one side and to the actuators on the other side. Bringing the pressurized hydraulic fluid supply apparatus into the flow separation state can reduce the energy loss of the pumps.
The apparatus of the above-mentioned Japanese Patent Publication No. Hei 7-92090, however, has the following drawbacks. In the apparatus of this kind, the pilot pressure for switching the flow merge/separation valve is often common to a pilot pressure for driving a directional control valve connected to the actuators. That is, the flow merging/separation is switched according to the lever operation by an operator. Here, let us consider a case where one wishes to perform, for example, a fine operation. The pilot pressure for driving the directional control valve during the fine operation is small. There is a large difference in load pressure between the first and second hydraulic circuits, and the flow merge/separation valve remains in the flow merging state because the pilot pressure for switching the valve is low. In this condition, the apparatus with the conventional configuration causes pump energy losses.
In the flow merging/separation control on the two hydraulic circuits, when a load pressure difference occurs, the hydraulic circuits may be merged, contrary to the above case, to equalize the loads and the delivery rates between the two pumps. At this time, if the switching is to be made by the pilot pressure, the following situation arises because of the low pilot pressure. Where the first pump is connected with a left-side travel motor and the second pump is connected with a right-side travel motor and actuators, if the actuators are driven with the hydraulic lines separated, the flow separation state fails to be switched to the flow merging state, resulting in an insufficient flow into the right-side travel motor causing the power shovel to advance in a curved path.
It is an object of this invention to provide a pressurized hydraulic fluid supply apparatus which can switch between the flow merging state and the flow separation state according to the driving state of a plurality of actuators and can keep the flow balance among a plurality of hydraulic pump lines in good condition.